1. Field of the Invention
The present invention relates generally to a method of adjusting the hardness of a titanium metal, and more particularly to a method of adjusting the hardness of a titanium metal obtained by a fused-salt electrodeposition method.
2. Description of the Prior Art
In general, it is well known in the art that the hardness of metal depends greatly on an amount of impurities or additives in the metal. In the case of titanium metals, if an interstitial atom such as oxygen, nitrogen and the like is contained in titanium, the hardness of the titanium is greatly increased as compared with the hardness of titanium containing no interstitial atoms. Accordingly, titanium metal which contains oxygen or the like to increase its hardness and its mechanical strength is effective from the practical standpoint and hence is widely used. However, when too much oxygen is contained in itianium the hardness of the titanium is increased remarkably, but the ductility thereof is deteriorated. Thus, titanium containing too much oxygen cannot be used practically without considerable difficulty. Therefore, it is necessary to control or adjust the amount of oxygen to be contained in titanium in accordance with the purpose for which the titanium is to be used.
In accordance with the prior art, a titanium metal of the sponge type, which has been obtained by reducing titanium tetrachloride (TiCl.sub.4) with metallic magnesium or sodium, is heated and fused with a suitable amount of titanium oxide (TiO.sub.2) to produce titanium containing a predetermined amount of oxygen and, hence, having a predetermined hardness. However, in the foregoing process, the reduction of the titanium tetrachloride with metallic magnesium or sodium is a batch process which has to be performed in a sealed container, and considerable difficulty is experienced in separating the resulting titanium metal from the magnesium chloride or sodium chloride that is a by-product of the reaction.
In order to avoid the above problems, the present inventors along with others have previously proposed, for example, as disclosed in Japanese Patent No. 726,754 and Japanese Patent Applications No. 107,500/74 and 141,960/74, an electrolytic method by which a titanium metal is directly obtained by electrodeposition on an electrode as a compact block or plate having a smooth surface. However, in such known method for obtaining titanium metal by electrolysis, it is difficult or almost impossible to adjust the hardness of the electrodeposited titanium.